AP Biology 8.6: Biodiversity and Ecosystem Change

The common wild oat is native to regions of Europe and Asia but is an invasive species in central California grasslands. In California, the common wild oat has almost completely replaced some species of native bunchgrass. Researchers found that aphids, a type of small insect that often carries plant viruses, have a much higher reproductive rate in grasslands that include the common wild oat than in grasslands composed of only native bunchgrass species. Additionally, the viruses carried by the aphids appear to affect only the native bunchgrasses and not the common wild oat. Native bunchgrasses infected by the virus have much higher death rates than do native bunchgrasses that are not infected.

Describe the change in the resilience of an ecosystem when there is a decrease in the number of species.

Buffelgrass, an invasive grass species in southwestern desert ecosystems, is threatening the

saguaro cactus, a keystone species in these ecosystems. Buffelgrass is drought-tolerant and can

survive wildfires. However, the dry buffelgrass also acts as fuel for wildfires, causing the fires

to be more severe. Older saguaro cacti can survive wildfires; however, many of the young cacti

cannot.

Scientists conducted an experiment to determine whether they could control the abundance

of the buffelgrass population. The scientists identified several native grass species that, when

grown with buffelgrass, might reduce the abundance of buffelgrass. They grew buffelgrass

in the presence of several different native grass species in greenhouses, in either nondrought

(watered every 3 days) or drought (watered every 9 days) conditions. After twelve weeks, they

measured the height and dry weight of the buffelgrass in each treatment group.

Describe the effect that removing a keystone species will have on an ecosystem.